In an image forming system, there is generally provided a sheet feeding means that feeds out a sheet to an image forming section one by one from a sheet-feeding table that stores sheets. Though various types of sheet feeding means have been put to practical use as this sheet feeding means, an FRR sheet feeding system (Feed and Reverse Roller system) or an air suction belt sheet feeding system is mainly used.
A sheet feeder of the FRR system is constructed with a pick-up roller that comes in contact with the uppermost portion of sheets placed on a sheet-feeding table, and draws a sheet out of the sheet-feeding table by frictional force, a feed roller that feeds out the sheet drawn out to a sheet conveyance path, and a separation roller that contacts the feed roller at a prescribed pressure and is driven to rotate in the direction opposite to the sheet conveyance direction through torque limiter.
In the sheet feeder, when the separation roller is in contact with the feed roller and when only one sheet passes through a nip between both rollers, the separation roller is driven by the feed roller through a slip of the torque limiter to rotate in the sheet conveyance direction to feed out the sheet in the conveyance direction. However, when two or more sheets pass through the nip between both rollers, rotation drive torque of the separation roller overcomes the frictional force between the two sheets to cause them to slide each other and thereby to return the sheet that is in contact with the separation roller toward the sheet-feeding table, whereby, the sheets which are in close contact are separated.
In the sheet feeder of an air suction belt sheet feeding system, on the other hand, air blows against an edge portion of the sheet from a blower fan to lift the leading edge of the sheet and to separate the sheets simultaneously. The lifted sheet is vacuumed by a suction belt or by a suction roller to be fed out to a sheet conveyance path, and when two or more sheets are vacuumed and conveyed, a sheet separating pad made of rubber material (advance-resisting member) that is provided to close a sheet path prevents sheets other than the topmost sheet from being fed forcibly, in the structure.
In particular, compared with other methods, the sheet feeder of the FRR system has versatility for a range of sheet types to be applied, and various improvements and proposals have been made for improving reliability for sheet feeding functions relating to this method. For example, in Japanese Unexamined Patent Publication No. 05-32356, there is proposed an apparatus wherein a double feeding detection sensor is provided, and results of the detections of the detection sensor are fed back to adjust automatically a nip pressure between the feed roller and the separation roller. In Japanese Unexamined Patent Publication No. 08-188286, there is proposed an apparatus wherein a sheet thickness detection sensor is provided, and an object is to enhance reliability of sheet feeding by controlling separately the speed of rotation of the feed roller and the speed of rotation of the separation roller, based on information of the detection.
The sheet feeder of the FRR system has the following problems. In general, the separation roller tends to be worn away because it is driven to rotate constantly in the direction opposite to the sheet conveyance direction while it keeps touching the feed roller at a prescribed pressure, thus, it has a tendency to be lower in terms of durability, compared with an ordinary rubber roller. When a sheet having lots of paper dust or a sheet which has been subjected to offset printing passes through the rollers, paper dust, dusting powder and ink sediment adhere the pickup roller, the feed roller and the separation roller to lower friction of rollers, which sometimes causes conveyance troubles. Further, when a thin sheet passes through, the sheet is sometimes damaged by actions of these friction rollers.
On the other hand, when a thin sheet and a sheet having lots of paper dust pass through rollers, an air suction belt sheet feeding system employing no friction roller is effective. However, a sheet feeding apparatus of the air suction belt sheet feeding system has the following problems. For example, when feeding a thick sheet, the suction power for conveying the sheet needs to be great, resulting in a large-sized apparatus, which is a weak point. Further, when feeding a thick sheet, the sheet sometimes fails to pass through a sheet separating pad made of rubber material provided to close a sheet path, which sometimes requires to change position setting for the sheet separating pad depending on a type of the sheet and on weight.
In the sheet feeding apparatus, conditions for sheet feeding and separation are changed variously depending on a type and weight of the sheet or on environmental circumstances, as stated above. Therefore, in the case where the sheet feeding apparatus is used on a multipurpose basis, and a sheet feeding means of an FRR sheet feeding system or a sheet feeding means of an air suction belt sheet feeding system is employed independently, the problems of double feeding or sheet feeding troubles cannot be solved sufficiently, and securing of reliable sheet feeding is sometimes difficult.
A sheet feeding apparatus employing both the FRR sheet feeding system and an air assist system to supply air from a side guide of a sheet-feeding table and to separate a sheet, is effective and has been put to practical use. However, this sheet feeding apparatus has problems that when a sheet having lots of paper dust or a sheet which has subjected to offset printing passes through the rollers, paper dust, dusting powder and ink sediment adhere the rollers, to lower friction of rollers, which causes conveyance troubles, in addition to establishment of conditions for a position for an air blast from the side guide and a rate of air flow, thus, a sheet feeding apparatus that is simple and is highly reliable has been demanded.
The invention has been proposed under the aforesaid circumstances, and an object of the invention is to provide a sheet feeding apparatus wherein a sheet feeding section of FRR system is arranged at the downstream side of a sheet feeding section of an air suction belt sheet feeding system, in view of the point that an FRR system has a weak point slightly for feeding a thin sheet and an air suction belt sheet feeding system has a weak point slightly for feeding a thick sheet, and high sheet feeding ability for a wide range of sheet types are realized by activating selectively sheet separating devices of both systems and durability of a friction roller is improved by controlling working hours for rotation of a separation roller.